The present invention relates to methods of forming an isolation trench in a semiconductor, methods of forming an isolation trench in a surface of a silicon wafer, methods of forming an isolation trench-isolated transistor, trench-isolated transistor, trench isolation structures formed in a semiconductor, memory cells and DRAMs.
Field-effect transistors (xe2x80x9cFETsxe2x80x9d) are used in memory structures such as dynamic random access memories (xe2x80x9cDRAMsxe2x80x9d) for controlling access to capacitors used to store charge representing information contained in the memories. In DRAMs, charge leakage effects necessitate periodic refreshing of the information stored in the memory. In turn, refreshing of the DRAM leads to increased power consumption and delays in memory operation. Accordingly, it is desirable to reduce charge leakage effects in DRAMs.
Additionally, it is desirable to minimize the area required for fabrication of the elements of memories such as DRAMs. Electrical isolation of various circuit elements from each other is required. In turn, electrical isolation requires some of the space used on the DRAM or other integrated circuitry. Various techniques have been developed to reduce the amount of area needed for electrical isolation structures. One technique for providing a high degree of electrical isolation while requiring relatively little space is known as shallow trench isolation.
One source of charge leakage in DRAMs is related to carrier generation-recombination phenomena. In general, lower dopant concentrations tend to reduce this source of charge leakage. However, other concerns tend to determine lower bounds for dopant concentrations.
The FETs used as access transistors determine some of these other concerns. The FETs need to be able to provide a high impedance when they are turned OFF, and a low impedance connection when they are turned ON. DRAMs and other memories use an addressing scheme whereby a wordline that is coupled to many transistor gates is selected, and at the same time a bitline or digitline that is coupled to many transistor drains is also selected. A FET that is located at the intersection of the selected wordline and the selected bitline is turned ON, and that memory cell is accessed. At the same time, many other FETs have a drain voltage due to the drains of these FETs being coupled to the selected bitline. These FETs exhibit some parasitic conductance as a result of the drain voltage. In some types of integrated circuits, a portion of that parasitic conductance is due to corner effects that are an artifact of using trench isolation techniques to isolate the FETs from one another and from other circuit elements.
These effects are described in xe2x80x9cSubbreakdown Drain Leakage Current in MOSFETxe2x80x9d by J. Chen et al., IEEE El. Dev. Lett., Vol. EDL-8, No. 11, November 1987; xe2x80x9cImpact Of Shallow Trench Isolation On Reliability Of Buried- And Surface-Channel Sub-xcexcm PFETxe2x80x9d by W. Tonti and R. Bolam, IEEE Cat. No. 0-7803-2031, 1995; xe2x80x9cShallow Trench Isolation For Advanced ULSI CMOS Technologiesxe2x80x9d, M. Nandakumar et al.; and xe2x80x9cShallow Trench Isolation Characteristics With High-Density-Plasma Chemical Vapor Deposition Gap-Fill Oxide For Deep-Submicron CMOS Technologiesxe2x80x9d, S.-H. Lee et al., Jpn. J. Appl. Phys., Vol. 37, 1998, pp. 1222-1227, which publications are hereby incorporated herein by reference for their general background teachings.
One method of reducing these parasitic conduction effects is to round the corner where the isolation trench meets the surface of the semiconductor material. This may be effected by oxidizing the surface of the silicon, as is described in the above-noted publications. However, this approach requires additional processing steps, which tend to result in reduced yield, among other things.
What is needed is a way to incorporate trench isolation together with FETs that does not increase processing complexity and that provides compact, low-leakage transistors in DRAMs and other circuitry.
In one aspect, the present invention provides a method of forming an isolation trench in a semiconductor. The method includes forming a first isolation trench portion having a first depth and having a first sidewall intersecting a surface of the semiconductor at a first angle. The method also includes forming a second isolation trench portion within and extending below the first isolation trench portion. The second isolation trench portion has a second depth and includes a second sidewall. The second sidewall intersects the first sidewall at an angle with respect to the surface that is greater than the first angle. A dielectric material fills the first and second isolation trench portions.
In another aspect, the present invention includes a method of forming an isolation trench in a surface of a silicon wafer. The method includes forming a mask on the surface, where the mask includes an opening and sidewalls, and etching the silicon surface using gases including CF4 and CHF3 in a ratio of CF4/CHF3=0.11 to 0.67 to form a first isolation trench portion.
In a further aspect, the present invention includes a trench-isolated transistor. The trench-isolated transistor includes first and second isolation trenches each disposed on a respective side of a portion of silicon. The first and second isolation trenches each include a first isolation trench portion having a first depth and having a first sidewall intersecting a surface of the silicon at a first angle. The first and second isolation trenches each also include a second isolation trench portion within and extending below the first isolation trench portion. The second isolation trench portion has a second depth and includes a second sidewall intersecting the first sidewall at an angle with respect to the surface that is greater than the first angle. The first and second isolation trenches are filled with a dielectric material. The transistor further includes a gate extending across the silicon portion from the first isolation trench to the second isolation trench, and source and drain regions extending between the first and second isolation trench portions and across the silicon portion. The source region is adjacent one side of the gate and the drain region is adjacent another side of the gate that is opposed to the one side.